A number of characteristics are expected of an accelerometer, principally a low size requirement, good sensitivity to an acceleration in a well-identified direction called the “sensitive axis”, a sensitivity as low as possible along the other axes, good linearity, good measurement precision, good mechanical strength, and low cost.
The mass fabrication technologies used in microelectronics are particularly suited to obtaining a low production cost and the cost is then directly linked to the size of the micromachined component. Depending on the application envisaged for the accelerometer, optimization of the various characteristics above will be attempted and the necessary compromises will be made, for example a compromise between small size and good sensitivity.
A micromachined accelerometer generally comprises a movable seismic mass (or proof mass) connected to a substrate in which it has been machined by an elastic connection. A resonant or vibrating beam (or several such) is fixed between the mass and the fixed substrate and undergoes directly or indirectly, possibly even with mechanical amplification, the forces connected with the movement of the seismic mass during an acceleration. The beam is vibrationally excited (for example through the effect of electrostatic forces between opposite conductive zones) by an electrical excitation circuit; the excitation circuit is an oscillating circuit which the electrically conductive beam is part of. The beam behaves like a movable capacitor plate undergoing electrostatic forces and causing variations in capacitance. The electrical oscillation frequency that is established in such a circuit is a mechanical resonant frequency of the beam. This frequency depends on the tension (or the compression) of the beam in the longitudinal direction; this tension results from the force exerted by the seismic mass, hence from the acceleration undergone by the mass. Measurement of the frequency of electrical oscillation enables the acceleration undergone by the mass to be determined.
The oscillation frequency f is of the form f=f0(1+P/Pc)1/2, where f0 is a natural oscillation frequency in the absence of tensile or compressive stress, P is the tension exerted on the beam (signed, i.e. a tension or a compression according to whether P is positive or negative), Pc is a fixed value linked to the geometry of the beam and to the material constituting it.
In the prior art many constitutions of accelerometer have been proposed. The following may notably be cited:
Pendulous accelerometers in which the mass rotates about a hinge that connects it to the substrate.
These accelerometers are not generally high performance for small accelerations, particularly if there are several resonant beams, due to mechanical couplings between the beams. In addition, the rotations carried out during the movement of the mass have the effect that the forces exerted by the mass on the resonant beam are not exactly on the axis of the beam; this results in interference force components and a poor use of the energy of movement applied by the mass to the beam: part of the energy is used to deform the beam by bending, which is not expedient to the measurement; linearity and sensitivity are not optimum.
Some structures locate the resonator in such a way that the axis of the beam is parallel to the sensitive axis and perpendicular to the line that connects the point of rotation of the mass and the point of application of the force on the beam. However, this often results in an unfavorable arrangement from the point of view of compactness and there are difficulties with industrial production.
Such accelerometers are described, for example, in the following publications:    EP 0 331 557 B1, U.S. Pat. No. 6,941,809 B2, FR 2 784 752 A1, WO 98/53328 A1;    Young Ho Seo and Young-Ho Cho: Design, Fabrication, Static Test and Uncertainty Analysis of a Resonant Microaccelerometer, in Sensors and Materials, vol. 14, no. 2 (2002), pp. 91-108;    O. Le Traon et al.: The Via Vibrating Beam Accelerometer: A New Quartz Micromachined Sensor, in 1999 Joint Meeting EFTF-IEEE IFCS, pp. 1041-1044;    M. Aikele et al.: Resonant Accelerometers with Self-Test, in Sensors and Actuators A 92 (2001), pp. 161-167;    Peter H. Lafond: Modeling for Error Reduction in Vibrating Beam Accelerometers, in IEEE 1992, Position Location and Navigation Symposium, pp. 126-132;    Roessig et al.: Surface Micromachined Resonant Accelerometer, in Transducers 97 IEEE, pp. 859-862.
So as to make up for the kinematic errors of the oscillating structures other structures have also been proposed in which the seismic mass moves parallel to itself (therefore in translation and not in rotation about a hinge). The resonator is then most often connected directly to the mass, which prevents an amplification effect on the force due to the acceleration being obtained. Sensitivity is therefore limited. These structures are in general bulky, especially when differential operation with two resonant beams is desired (one beam being subjected to a tensile force while the other is subjected to a compressive force).
Other structures with translational movement of the mass have been proposed with an amplification structure placed between the mass and the resonant beam. This amplification structure comprises a force amplification lever. The force exerted by the mass on the lever at one point is transmitted with an amplification factor to another point of the lever. These structures are complex: they further require an intermediate part and an articulation between the lever and the resonant beam. They are bulky, particularly if they must operate differentially with two resonant beams working, one under tension the other under compression, during a movement of the seismic mass.
Such accelerometers, provided with mechanical force amplification means are described, for example, in the following publications:
FR 2 848 298 A1: this structure has a force amplification factor linked to the value of the tangent of an angle which is difficult to control precisely in industrial fabrication.
U.S. Pat. No. 5,969,249 and the article by Ashwin A. Seshia et al.: A Vacuum Packaged Surface Micromachined Resonant Accelerometer, in Journal of Microelectromechanical systems, vol. 11, no. 6, December 2002. This structure has a lever amplification effect, but it is particularly bulky and the articulation of the levers to the substrate is necessarily fairly rigid because there is not enough room to put in a more flexible articulation without further increasing the size requirement.
The accelerometers that will be considered here are resonant accelerometers having a seismic mass capable of moving mainly in translation along a sensitive axis lying in the plane of the substrate in which the mass is machined (in contrast to other accelerometers in which the sensitive axis is perpendicular to the plane of the substrate). It will be assumed in the following that the geometry of the accelerometer is defined in an orthogonal reference frame Ox, Oy, Oz specific to the accelerometer; the axis Oy, in the plane of the substrate, is the sensitive axis; the axis Ox, also in the plane of the substrate, is perpendicular to Oy; the axis Oz is perpendicular to the plane.
In these accelerometers the seismic mass is suspended from anchor points fixed in relation to the substrate by means of elastic connections having a low stiffness in the direction of the sensitive axis Oy and a high stiffness along the two other axes Ox and Oz. It can therefore only move, practically, in the Oy axis. Its movement is limited within the range of accelerations to be measured by the return force exerted by the elastic connections.
The seismic mass is connected through a force amplification structure to a resonator on which it exerts a tension or a compression. The resonator is in general a simple resonant beam or an assembly of two parallel, mechanically coupled, resonant beams (tuning-fork resonator). It is also micromachined in the substrate and in general has one end anchored in the substrate. The mechanical connection between the mass and the beam is such that the movements of the mass along Oy exert a tension or a compression on the beam in the longitudinal direction of the beam. The resonator is associated with means for exciting a vibration and with means for measuring the resultant vibration frequency.